Human diarrhea is the 3rd leading cause of infectious deaths worldwide causing approximately 2,000,000 fatalities per year. In most studies of diarrhea etiologies, on average about 40% of cases cannot be attributed to any known microbial agent. Among the established viral causes are rotaviruses, noroviruses, sapoviruses, enteric adenoviruses and astroviruses. Until very recently, human astroviruses were thought to be limited to 8 closely related serotypes (hereafter referred to as "classic" human astroviruses), which are estimated to be responsible for approximately 10% of diarrhea cases. In the past two years, two novel astroviruses have been discovered in the stool of patients with diarrhea. These viruses, astrovirus MLB1 (MLB1) and astrovirus MLB2 (MLB2), are highly divergent from the classic human astroviruses as well as all other known astroviruses. MLB1 and MLB2 share about 80% amino acid identity to each other. To date, both MLB1 and MLB2 have been detected in North America and India;in addition, MLB1 has also been described in Australia and Africa demonstrating that these viruses have a worldwide distribution. These two viruses define a novel clade of astroviruses, whose epidemiology and potential role in human diarrhea or other diseases has been completely overlooked in prior studies. The focus of this grant is to begin to address the question "Are the newly discovered astroviruses MLB1 and MLB2 responsible for some fraction of the 40% of unexplained diarrhea?" Towards this end, this R21 grant aims to define the epidemiology and seroepidemiology of these two novel viruses. Specifically, in Aim 1 the potential association of MLB1 and MLB2 with diarrhea will be determined by case-control analysis of a longitudinal birth cohort from Vellore, India. In Aim 2, age-stratified serum collections from both North America and India will be analyzed to define the seroprevalence of MLB1 and MLB2. PUBLIC HEALTH RELEVANCE: Two new species of astroviruses have been recently discovered in patients with diarrhea. The goal of this project is to begin to understand whether these viruses might be responsible for causing the diarrhea. The first step is to see if there is any correlation between presence of the virus and diarrhea symptoms.